The recycling of receptors, pumps and channels to the plasma membrane is an essential process in all epithelial cells for maintaining the repertoire of proteins necessary for the maintenance of normal polarized cell physiology. The ability of the cell to react to signals, to internalize nutrients, and to maintain ion equilibrium and secretion are all determined by the presence of active membrane proteins at the plasma membrane surface. Investigations over the past five years have established the importance of the Rab11 family of small GTPases in regulating plasma membrane recycling. In epithelial cells, Rab11a and Rab25 regulate apical recycling and transcytosis without affecting basolateral recycling. Rab11a is particularly enriched in gastric parietal cells, which support a massively amplified apical recycling system responsible for the regulated recycling of H/K-ATPase to the apical secretory canaliculus. The amplification of the Rab11acontaining recycling system has allowed us to identify rapidly a family of 6 Rab11 interacting proteins and the motor protein responsible for movement of vesicles out of the recycling system, myosin Vb. It is now clear that the Rab11 family members are part of a complicated assembly of protein regulating passage of cargoes through and out of the recycling system. We have hypothesized that Rab11a forms the nidus for the assembly of multiprotein complexes that regulate and mediate plasma membrane recycling. To address this hypothesis we will pursue four specific aims: First, we will isolate and identify the components of endogenous Rab11-coordinated complexes associated with recycling system vesicles. Second, we will examine the role of Rab11-FIP2 in regulating trafficking through the recycling system. Third, we will investigate the role of Rab11-FIP1 in the modulation of Rab11 function, In particular by focusing on the identification of proteins that interact with Rab11-FIP1. Fourth, using in vitro transfection of primary cultures and transgenic mouse models, we will examine the function of Rab11 family members and their modifiers on gastric parietal cell function. These studies should allow us to identify the critical components of Rab11acoordinated protein complexes regulating the plasma membrane recycling systems.